In the field of geophysical exploration, particularly seismic exploration, it has been found useful to place equipment deep into boreholes (well below the earth's surface) for a variety of reasons, such as measuring seismic energy, micro earthquake recording, determination of fracture orientation or geometry in oil well hydrofracturing, etc.
For example, seismic receivers, or geophones, may be lowered downhole to measure the seismic signals created from explosive shots on the surface or, in the case of crosshole technology, deep within a nearby wellbore.
A typical tool of the relevant art includes the following elements in a single housing: sensors, such as geophones, that convert mechanical vibrations into electric signals; associated electronics; a clamp that wedges the tool against the borehole wall; and a motor that actuates the clamp.
During acquisition of seismic data, the detector is lowered into a borehole, which borehole is generally filled with a fluid such as water, oil, drilling fluid or fracturing gellant. It is then clamped at a desired depth. Seismic waves are created by conventional sources and detected by the tool. The tool is then placed at a different depth, and the process repeated. In the most common configuration, data can be recorded by only one detector unit at one depth at a time. Recently, multiple downhole tools have been introduced to obviate repeated relocation of a single tool.
Many of these single downhole logging and seismic tools contain apparatus which creates a constant hydraulic pressure differential relative to hydrostatic borehole pressure. This means that the amount of pressure in the hydraulic system is always a certain set amount over the hydrostatic borehole pressure, which borehole pressure varies with the depth at which the downhole pump is operating. Typically, this hydraulic pressure is used to operate a clamp, usually on an "arm," to secure the tool to the wall of the borehole. Generally, pressures of 200 to 500 psi above the varying hydrostatic pressure are needed to provide sufficient force for a firm clamp.
One type of downhole tool that uses a hydraulic pressure generating apparatus is a wall locking geophone as described in the patent to Gustavson et al (U.S. Pat. No. 3,777,814). This pump consists of a dual hydraulic system to protect the delicate components of the pump from the pressure of the borehole fluid. The first hydraulic system includes an electric motor connected to a piston, both of which are located in a pressure-tight bay, and a second piston in a chamber exposed to borehole pressure. The second hydraulic system includes a third piston which is mechanically coupled to the second piston in the first hydraulic system and which generates the differential hydraulic pressure to clamp one geophone assembly to the borehole wall. Such hydraulic systems are typical in the art.
Additional problems are presented, however, when the downhole hydraulic pump is required to service multiple downhole tools. An example of this case is presented in U.S. patent application 07/652,333, wherein multiple downhole geophones are used simultaneously. The hydraulic pumps of the related art can supply the pressure to clamp a single unit, but cannot sufficiently pressurize the large volume of hydraulic fluid required to clamp multiple units. To adapt the downhole pump of Gustavson to this service would require the use of unfeasibly long pistons.